The present invention relates to apparatus and methods for determining the density of test materials, and more particularly to an aparatus and method for directly measuring from a composite material comprised of a relatively thin top layer applied to an overlying base material, the density of the top layer.
Nuclear radiation gauges for determining the density of soil and asphaltic materials are well known, as described for example in U.S. Pat. No. 2,781,453. Such gauges employ the phenomenon of Compton scattering of gamma rays and are known by those skilled in the art as "scatter" gauges.
The gauges most commonly used heretofore for measuring the density of soil, asphalt, and other materials are most effective at measuring densities of materials over depths of approximately four to six inches. When the thickness of the test material is at least four to six inches, the prior gauges have been highly successful. However, as the thickness of the test material decreases, increasing difficulty is encountered due to the influence of the underlying material.
With the increasing cost of paving materials, the practice in maintaining and resurfacing paved roadbeds has become one of applying relatively thin overlays, e.g. on the order of about one to two inches, over the existing roadbed. With thin layers of this thickness, prior nuclear density gauges of the type noted above are ineffective for measuring the density of the overlay applied. More particularly, such gauges are not capable of directly measuring the density of layers having a thickness less than about four inches. Because of the depth of penetration of the gamma rays, the gauge "sees" through the thin overlay so that the underlaying pavement substantially influences the gauge reading.
Recognizing this limitation of prior density gauges, efforts were made in the mid 1970's to establish a procedure for determining the density of thin overlays utilizing the then existing gauges. A nomograph was developed which allowed approximation of the density of the thin overlay. However, in order to obtain the density of the overlay by the nomograph technique, it was necessary to know both the density of the underlaying base material and the thickness of the overlay. The technique was as follows. First, the operator determined the density of the base material by taking nuclear density tests of the existing roadbed. Second, after the overlay pavement was applied and compacted, the overlay thickness was determined by taking a core sample, or similar operation. Third, density tests were performed on top of the overlay. With the density measurement from the first test, the overlay thickness measurement and the density data from the second test, the density of the overlay could be approximated by reference to the nomograph.
A similar technique for determining the density of a thin overlay is described in U.S. Pat. No. 4,389,136. As in the nomograph technique, it is necessary to first determine the density of the base prior to application of the overlay, and thereafter to determine the thickness of the overlay and the density of the composite material.
A significant drawback of both the prior art nomograph technique and the technique described in the above-referenced U.S. Pat. No. 4,389,136 is that the underlying pavement may be further compacted when the overlay is compacted, thereby introducing an inconsistency between the gauge reading of the underlying pavement density and its actual density after application of the overlay. Furthermore, it has proven difficult to take the second density reading (after the overlay is applied) at precisely the same location as the first reading. Also, the thickness of the overlay may vary between the sample location and the location where the nuclear gauge is placed for testing. In addition to the above problems, and perhaps more importantly, these techniques require multiple steps, performed both before and after application of the overlay, and usually also require a destructive thickness measurement of the overlay.
Recognizing the shortcomings of the prior methods, a method and apparatus is described in commonlyowned copending U.S. patent application Ser. No. 477,820 filed Mar. 22, 1983, now U.S. Pat. No. 4,525,854, by which the density of a thin overlay may be directly determined. In one aspect, this apparatus comprises a radiation source for emitting radiation into a relatively thin material and any underlying substrate material that is present, and detector means designed for obtaining separate and distinct measurements of scattered radiation at a plurality of detector locations. Because of the design, the separate and distinct radiation measurements are weighted toward the physical characteristics as they exist at different depths in the thin material and underlying substrate and provide independent data that, when interrelated with derived mathematical relationships, serve collectively to determine values for the physical characteristics of the thin layer and substrate.
In the preferred embodiment illustrated in this copending application, the gauge includes a source of gamma radiation and three radiation detectors that provide three independent total radiation counts. The three independently derived total radiation counts, when substituted into three empirically derived simultaneous mathematical equations (each having three unknowns), enable all three unknowns to be calculated. The apparatus may include a microprocessor that incorporates a fixed set of instructions for performing the solution of the three simultaneous equations.
While the techniques described in the abovereferenced copending application have been successfully employed for obtaining direct measurement of density of a thin layer of test material, the use of three detectors and the solving of simultaneous equations impose limitations on the cost and efficiency of the gauges employing these techniques.
The present invention represents an improvement over the techniques described in the aforementioned copending application in that a simplified and more economical means and method are employed for achieving direct measurement of the density of a thin overlay.